ERR_PACKAGE_PATH_NOT_EXPORTED with @modelcontextprotocol/express

Describe the bug

I am getting a ERR_PACKAGE_PATH_NOT_EXPORTED with message No "exports" main defined in C:\Users\myusersmyproject\node_modules\@modelcontextprotocol\express\package.json when running the @modelcontextprotocol/express example as per the usage guide here

import { createMcpExpressApp } from '@modelcontextprotocol/express';
const app = createMcpExpressApp();

This looks similar to #726 , #427

To Reproduce

Node version 24.14.1 LTS on Windows 11.

Project initialized with

npm i -D typescript
npx tsc --init

verbatimModuleSyntax commented out in tsconfig

Expected behavior

Having no error

Additional context

Dependencies:

{
  // ...
  "dependencies": {
    "@modelcontextprotocol/express": "^2.0.0-alpha.2",
    "@modelcontextprotocol/node": "^2.0.0-alpha.2",
    "@modelcontextprotocol/server": "^2.0.0-alpha.2",
    "express": "^5.2.1"
  },
  "devDependencies": {
    "@types/express": "^5.0.6",
    "typescript": "^6.0.2"
  }
}

fix(express): add CJS exports to resolve ERR_PACKAGE_PATH_NOT_EXPORTED

I have set "type": "module" in package.json and "module": "NodeNext", "moduleResolution": "NodeNext" but now I am getting:

Error: Cannot find package '@cfworker/json-schema' imported from /home/moreillon/dev/mcp-sdk-test/node_modules/@modelcontextprotocol/server/dist/src-IKPjmxu7.mjs
code: 'ERR_MODULE_NOT_FOUND'

I’m seeing what looks like another published artifact issue in the 2.0.0-alpha.2 packages, this time with Deno/JSR resolution.

Minimal repro:

// repro.ts
import "npm:@modelcontextprotocol/server@2.0.0-alpha.2";

deno run --reload --node-modules-dir=none repro.ts

This fails with:

error: Could not find package '@cfworker/json-schema'
from referrer .../@modelcontextprotocol/server/2.0.0-alpha.2/dist/src-IKPjmxu7.mjs
(@modelcontextprotocol/server@2.0.0-alpha.2).

I checked the published tarball, and dist/src-IKPjmxu7.mjs contains a top-level static import:

import { Validator } from "@cfworker/json-schema";

But @cfworker/json-schema is published as an optional peer dependency, not a regular dependency, so Deno does not include it in the resolved npm dependency graph.

From current main, this looks like it may already be fixed because the CF worker validator no longer appears to be pulled into the main server bundle. If that is correct, could you please publish a new alpha with the current packaging fixes? 2.0.0-alpha.2 is currently not usable from Deno/JSR even for a plain server import.

Closed as already fixed in PR #2405, published in 2.0.0: every v2 package now ships dual ESM and CommonJS builds with a require condition in its export map. See the express package.json and tsdown config. Verified against the published @modelcontextprotocol/express@2.0.0: require('@modelcontextprotocol/express') resolves createMcpExpressApp.

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ERR_PACKAGE_PATH_NOT_EXPORTED with @modelcontextprotocol/express #1858

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ERR_PACKAGE_PATH_NOT_EXPORTED with @modelcontextprotocol/express #1858

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maximemoreillon commented on Apr 17, 2026

yasaichi commented on May 8, 2026

mattzcarey commented on Sep 11, 2026

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